Most modifications of the Hubble classification system (e.g., de
Vaucouleurs 1959a, van den Bergh 1960 a,b,c, Elmegreen & Elmegreen
1982) have added features to the original Hubble scheme. However,
Morgan (1958, 1959a) devised a classification system that was even
simpler than Hubble's original scheme. He classified galaxies mainly
on the basis of their central concentration of light. This contrasts
with the Hubble system which is based on both the central
concentration of light and on the morphology of their spiral
structure. Abraham et al. (1994) have shown that a one-dimensional
classification system, such as that introduced by Morgan, is
particularly well suited to classification of digital images by
computers. Furthermore, Morgan's ``Yerkes'' system avoids the conflicts
that occasionally arise between the classifications based on arm
morphology and those relying on central concentration of light. An
example of an object where such a conflict arises is the galaxy
NGC 4866 which is assigned to type Sa, even though
it has a rather small
core. Abraham et al. (1994) also find that Morgan's one-parameter
classification system can be used to provide an adequate description
of the morphologies of galaxies in very rich clusters. In such
clusters the Hubble system provides little resolution because almost
all galaxies are of types E and S0.